Rob Zombie is billed over Marilyn Manson at Rock on the Range.
Manson headlined Mayhem 09 over Slayer.
Slayer headlined a tour over Zombie last summer.

So they're all pretty close in popularity. This could really go either way for this tour. The RotR billing is most recent, but Manson just had an album come out. I think Manson may have the slight edge due to that alone, but it will still be a co-headliner obviously either way.

I don't see Zombie not headlining if not for the fact alone that he has a huge stageshow. I don't see them seeing it up then taking it down right away for Manson.

Remember also that on the Hell on Earth tour with Slayer last year it seemed like Slayer closed over half of the shows on the tour, plus the stage set up from what I saw in youtube video wasn't super complex

That being said I think Zombie deserves to headline over Manson, although if this comes to Denver and Manson is closing I get to get out early

Manson is selling less albums than Disturbed, Godsmack, LoG... he should be main support. He is selling the same amount of albums now that Mastodon is. Its a good amount but not good enough imo to headline over Zombie, especially with Zombie having part of Manson's most popular line up in his own band now. Manson was more notorious for a period of time in the 90's but White/Rob Zombie has had a more successful career imo. Manson has had a successful career & I like alot of his music but I think Zombie has alot more fans of casual hard rock that will turn out to see him compared to Manson's smaller but more loyal following.